<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="https://cdn.jsdelivr.net/npm/vue@2.6.14"></script>
</head>
<body>
  <div id='root'></div>
  <script>
    // mixin
    const empty_mixin = {
      filters: {
        empty: (value) => {
          return value || '--'
        }
      }
    }

    new Vue({
      el: document.querySelector('#root'),
      mixins: [ empty_mixin ],
      data: {
        user: ''
      },
      template: ` 
        <h1>hello, {{ user | empty }}</h1>
      `
    })
  </script>
</body>
</html>